Method of improving the drainage of cellulose fiber suspensions with polymers of n-vinyl-n-methyl-formamide

ABSTRACT

DEWATERING OF AQUEOUS CELLULOSE FIBER SUSPENSION IN THE FORMATION OF PAPER IS IMPROVED BY ADDING TO THE SUSPENSION FROM 0.05 TO 4% OF A WATER SOLUBLE POLYMER CONSISTING ESSENTIALLY OF UNITS DERIVED FROM N-VINYL-N-METHYLFORMAMIDE, SUCH AS N-VINYL-N-METHYL FORMAMIDE, HAVING 60-100% OF THE FORMIC ACID RADICALS OF THE POLYMER SPLIT OFF BY ACID HYDROLYSIS.

United States Patent int. c1. 1021a 3/00; mm 3/40 US. Cl. 162-168 5Claims ABSC'I OF THE DISCLUS Dewatering of aqueous cellulose fibersuspension in the formation of paper is improved by adding to thesuspension from 0.05 to 4% of a water soluble polymer consistingessentially of units derived from N-vinyl-N-methylformamide, such asN-vinyl-N-methyl formamide, having 60l00% of the formic acid radicals ofthe polymer split off by acid hydrolysis.

The present invention relates to an improved method for dewateringcellulose fiber suspensions suitable for the manufacture of paper.

In the production of paper, card board and similar water-scooped paperwebs-hereinafter called papera cellulose fiber suspension Which maycontain more or less rosin size and mineral constituents (fillers)depending on the product desired, is drained on a continuously movingWire cloth. In this process, the free water passes and drops off thepaper web whereas the rest of it is removed by suction and subsequentdrying, for example, on drying rollers. A substantial factor in theproduction of paper is the rate at which the water can pass through thepaper web or can be sucked off. The drainage rate of the cellulose fibersuspensions depend, to a large extent, on the composition of thesuspension, on how finely beaten the fibers are (degree of fineness) andon the mesh width of the wire cloth. By adding certain chemicalsubstances dewatering can be accelerated without increasing the amountof fibers that drop through the wire cloth. Quick dewatering meanseither low drying costs or increase in production.

It is especially interesting to add draining agents of the chemical typein the processing of waste-paper or highly beaten cellulose (parchmentsubstitute) since, in this case, dewatering is considerably reduced bymajor proportions of finest fibers and slimy material.

It is known that cellulose suspensions can be dewatered better and morequickly by adding bivalent metal ions, especially calcium chloride (c.f.J. C. MacGugan, Paper Trade Journal, January -1962, p. 22). According toW. C. Cohen, et al., Proc. Austr. Pulp Paper Ind. Techn. Assoc. 3(1949), p. 72, this phenomenon resides in the neutralisetion of theanionic group of the fibers by positive metal ions. This leads to theformation of fiber agglomerates that permit an easier removal of waterfrom the fibrous material suspension.

Unfortunately, the water hardness plays an important part in thisprocess. Thus, it appeared that the draining effect decreases if thewater hardness increases. However, since the paper producer is, in mostcases, dependent on river or pond water or on underground Water ofmedium or high hardness, the use of calcium chloride is limited to a fewcases where soft water is actually available.

The draining effect of calcium chloride is, moreover, adversely affectedby the presence of rosin size and alumi- 3,597,3M Patented Aug. 3, 197inium sulfate so that its use is only economical with unsized papers. Inaddition, water containing calcium chloride promotes the corrosion ofsteel and brass slightly more than does usual water for industrial use.

German Pat. No. 1,169,131, furthermore, discloses that polymerizationproducts based on ethylene imine, for example polyethylene imine, aresuitable for accelerating dewatering of cellulose suspensions. Theseproducts have, in addition to other effects, a retaining effect andthereby reduce the amount of solid materials in the slurry water (cf. H.Wilfinger, Das Papier 2, (1.948) p. 265). A lower waste-water contentgenerally involves a better clarification of the sewage water and thusthe paper producer is in a better position to meet the requirementsimposed by the law as to the introduction of waste water into effluentchannels.

The above-cited draining agents based on polyethylene imine, however,exhibit a series of drawbacks. For example, it appears that papers thathave been made from those suspensions treated with polyethylene iminehave an increased tendency to yellowing. Moreover, pure polyalkyleneimines are sensitive to sulfate-ions in an acidic medium, i.e. theireffectiveness decreases with increasing amounts of aluminium sulfate inan aqueous solution. H. Wilfinger, in Das Papier, 2 (1948), p. 265,explains this phenomenon with the formation of salt-like sparinglysoluble precipitates of polyalkylene imines in the presence of aluminiumsulfate. In numerous cases, however, the paper producer depends on theuse of aluminium sulfate since it is an inexpensive fiocculating agentand thus proved especially suitable in the clarification of sewagewater. As a precipitating and mordanting agent to be used for glues anddyestuffs it can hardly be replaced by other products showing similareconomical advantages. Furthermore, aluminium sulfate serves in manycases, to control the absorptive power of papers.

Attempts have been made to overcome the sensitiveness of polyalkyleneimines to sulfate-ions by using, instead of pure polyalkylene imines,condensation products of urea and 1,2-alkylene imines as auxiliaries.

It has now been found that dewatering of aqueous cellulose fibersuspensions to form paper on a paper machine wire cloth can be improvedby incorporating into the cellulose fiber suspensions water-solublehomoor copolymers of N-vinyl-N-methyl carboxylic acid amides of theformula CH CH-N (CH -COR in which R stands for hydrogen or the methyl orethyl radical, the carboxylic acid radicals of the polymers beingeliminated entirely or partially by acidic hydrolysis.

A substantial advantage of the auxiliaries of the invention is the factthat their effectiveness in an acidic medium is considerably lessafiected than that of, for example, polyethylene imine. Thisadvantageous property of the polymers used according to the invention isobviously due to their substantially weaker tendency to form sparinglysoluble salts with sulfate-ions in an acidic medium. An influence of thewater hardness on the draining effect, as observed with calciumchloride, can also not be established with the polymers used accordingto the invention. The draining effect of the polymers is also hardlyinfluenced by the presence of rosin size and aluminium sulfate. Comparedwith the condensation products of urea and 1,2- alkylene imines thepolymers used according to the invention, moreover, exhibit in a neutralmedium a superior draining effect on the cellulose fiber suspension. Itis, therefore, possible to use the polymers of the. invention fordewatering cellulose fiber suspensions within a wide range ofeffectiveness and independently of the Water hardness and of thepresence of aluminium sulfate or rosin size.

Another advantage of the polymers of the invention, especiallyestablished in practice, is the fact that, in despite their nitrogencontent surprisingly, they do not bring about yellowing of the paper.Owing to their excellent draining effect the products of the inventionare advantageously suitable for accelerating dewatering of slimybeatencellulose fiber suspensions or even of suspensions having a high portionof finest fibers or slimy material due to their origin, for example,from waste paper.

The compounds of the invention can also be used in suspensions thatcontain highly bleached cellulose fibers and, hence, would not permitthe use of compounds on the basis of polyalkylene imines owing to thetendency to yellowing. The presence of products of the inventioninfluences the effect of optical brighteners much less than do thosecompounds on the basis of polyalkylene imines.

Still another advantage of the products to be used according to theinvention is their very favorable effect of retaining filling andfibrous materials on the wire cloth during the sheet formation. Becauseof this advantageous retention valuable starting materials are, indeed,retained in the paper sheet to a large extent and the pulp slurries aremarkedly deposited thus permitting a better and easier clarification ofthe sewage water. It is, moreover, essential that the additives of theinvention do not deteriorate the absorptive power nor the surfacesmoothness of the final product.

The products to be used according to the present invention arewater-soluble polymers of N-vinyl-N-methyl carboxylic acid amides orcopolymers of these vinyl compounds with radically polymerizablecomonomers, the carboxylic acid radicals of which heve been eliminatedat least partially by hydrolysis with a strong mineral acid, for examplehydrochloric acid, nitric acid, hydrobromic acid, sulfuric acid orphosphoric acid, at an eleveated temperature. The polymers andcopolymers are prepared especially from N-vinyl-N-methyl carboxylic acidamides of lower alkane-carboxylic acids having 1 to 3 carbon atoms,especially N-vinyl-N-methylacetamide, N-vinyl-N- methylformamide ormixtures of these monomers. Preferred is N-vnyl-N-methylformamide sincethe hydrolysis of the corresponding homoand copolymers can be carriedout in an especially easy and technically simple manner at relativelylow temperatures of from about 50 to about 120 C. About 60 to 100%,preferably 70 to 93%, of the acyl radicals of the homoand copolymers areadvantageously split off by hydrolysis, these values indicating themol-percentage of nitrogen present in the form of free secondary aminogroups. The degree of hydrolysis is, conveniently, established bydetermining the content of basic nitrogen in the polymer. At the degreesof hydrolysis mentioned, the homopolymers have a content of basicnitrogen of from about 15 to 24% by weight, preferably 17 to 22.5% byweight. The carboxylic acid split off by hydrolysis and the mineral acidmay, if desired, be removed from the reaction mixture by evaporation,dialysis or by means of ion exchangers. The acids may, however, also beleft, in many cases, in the mixture without noticeably affecting theeffect of the polymers. The molecular weights of the partially orentirely hydrolized homoor copolymers to be used according to theinvention are advantageously within the range of from about 20,000 to5,000,000, preferably from about 100,000 to 2,500,000.

As auxiliaries to be used according to the invention there arepreferably mentioned the at least partially hydrolized products ofhomopolymers of N-vinyl-N-methyl carboxylic acid amides. As far ashydrolized products of copolymers of N-vinyl-N-methyl carboxylic acidamides are used, the proportion of the comonomers may be up to about50%, preferably up to 30%, calculated on the weight of the copolymer.

For preparing the copolymers any radically polymerizable compound may beused as comonomer of the N-vinyl- N-methyl carboxylic acid amides, forexample vinyl alcohol, vinyl esters such as vinyl acetate, otheropen-chain N- vinyl carboxylic acid amides, N-vinyl-lactams and, inparticular vinyl sulfonic acid, acrylic acid, methacrylic acid and theacrylic and methacrylic acid amides. The entirely or partiallyhydrolized homoor copolymers to be used according to the invention maybe prepared by the process disclosed in U.S. application Ser. No.766,942 filed Oct. 11, 1968 by Beermann, Schnabel and 'Ulmschneider,said application corresponding to German application F 53,845(renumbered P 17 20 737.3) filed Oct. 20, 1967.

The at least partially hydrolized homoor copolymers are introduced intothe fiber suspension, advantageously, at a moment between the lastbeating operation or the last thickening and the dewatering on the wirecloth of the paper machine. Too heavy a shearing stress of thesuspension, as caused by pumps and the like, should be avoided after theincorporation of the auxiliaries. The amount of the herein disclosedpolymeric auxiliaries to be incorporated into aqueous cellulose fibersuspensions is within the range of from about 0.05 to about 4%,calculated on the weight of the dry cellulose fibers. In most cases,however, amounts of from 0.05 to 0.3% by weight are sufficient.

The auxiliaries to be used according to the invention may beincorporated into fiber suspensions that have been obtained either fromfresh cellulose or from waste paper. The filling materials to beintroduced into the fiber suspensions may be the mineral substancescontaining silicates or sulfates as usually employed in the paperindustry, such as kaolin, China clay, talcum and heavy spar, as well astitanium dioxide.

The following examples serve to illustrate the invention but they arenot intended to limit it thereto.

EXAMPLE 1 Varying amounts of a poly-N-vinyl-N-methylformamide hydrolizedto an extent such that the basic nitrogen content amounts to 21% byweight (88 mol-percent of nitrogen present in the form of free secondaryamino groups) and the viscosity is 380 cp. in a 13.1% aqueous solution,are introduced into 1% aqueous fiber suspensions obtained by beating upnewsprint paper and showing a degree of fineness of about 65 SR. The pHvalue of the suspension is about 7. About 2 g. of atro fibers are takenfrom the suspension and placed into a Schopper-Riegler apparatus and thedewatering time is determined with the lower outlet pipe closed.

Suspensions, the pH-values of which are adjusted to 5 by means ofaluminium sulfate, are treated in the same manner.

In both cases the discharge time is also measured without adding aproduct and compared with polyethylene imine. The values obtained arecompiled in Table I.

EXAMPLE 2 Varying amounts of a partially hydrolized poly-N-vinyl-Nmethylformamide having a basic nitrogen content of 16.6% byweight (69 mol-percent of nitrogen present in the form of free secondaryamino groups) are introduced into the cellulose suspensions described inTABLE Iv Example 1 which have been adjusted to pH 5.0 by means ofaluminium sulfate. The reduction of the fineness degree of thesuspensions is measured in a Schopperatro cellulose, Dgwatenng Rleglerapparatus. For comparisons sake, varymg 5 Auxlhal'y Percent bywtvalueJ1me, Secs amounts of a condensation product of urea and ethylene M 149Condensation product of urea 3 7 0 97 1m1ne are examined under the sameconditions. The reand ethylene 1mm 74 sults are compiled in Table II andindicate a medium CopolymefofN'vmyl'N'methyl' T formamide and sodiumsalt of 0.3 7. 0 95 value obtained from 5 separate measurements.Vinylsulfonic acid 0.5 7.0 70 10 5 0 166 Condensation product of ureaTABLE H and ethylene imine Amount used Degree of copolymer of y yreferred to fineness formamide and sodium salt of atro cellulose, pHaccording vmylsulfonic acid 0 3 5. 0 130 Compound percent by wt. valueto S 0.5 5. 0 120 5.0 66.3 Condensation product of urea and 0.2 5.0 64.3We clam ethylene mime 8:2 2 8 223 1. In a process for dewatering anaqueous cellulose iggi g g figgggg g yy 8% 2-8 gig fiber suspension toform paper on a paper machine wire, 5 53:2 the lmprovement of Whichcomprises lncorporating 1nto EXAMPLE 3 A 1% aqueous fiber suspension isprepared from unbleached sulfite cellulose and unbleached wood pulp(60:40). The degree of fineness is 70.5 SR and, after acidification withaluminium sulfate to a pH of 5.0, it is 73 SR. Varying amounts of apoly-N-vinyl-N-methylformamide hydrolized to an extent such that thebasic nitrogen content amounts to 22.2% by weight (93 molpercent ofnitrogen present in the form of free secondary amino groups), areintroduced into samples of these suspensions and the dewatering time ismeasured as disclosed in Example 1 and compared with that ofpolyethylene imine and that of a condensation product of urea andethylene imine (Table III).

EXAMPLE 4 At a pH-value of 7.0 or 5.0, varying amounts of an almostentirely hydrolized copolymer of N-vinyl-N- methylformamide and thesodium salt of vinylsulfonic acid (ratio 70:30% by weight) areintroduced into cellulose suspensions as disclosed in Example 1. Theresults obtained are compared with those of a condensation product ofurea and ethylene imine. The dewatering time is measured as disclosed inExample 1.

the cellulose fiber suspension from about 0.05 to about 4%, referred tothe weight of the dry cellulose fiber portion of the suspension, of awater soluble polymer consisting essentially of units derived fromN-vinyl-N' methyl-formamide, having from to 100% of the formic acidradicals of the polymer split off by acid hydrolysis.

2. The process of claim 1 wherein from to 93% of the formic acidradicals of the polymer are split off by acid hydrolysis.

3. The process of claim 1 wherein in said suspension is incorporated atleast a partially hydrolyzed polymer consisting of a polymer having atleast 70% of its recurring units derived fromN-vinyl-N-methyl-formamide.

4. The process of claim 1 wherein in said suspension is incorporated atleast a partially hydrolyzed polymer consisting of a polymer having at:least 70% of its recurring units derived fromN-viny'l-N-methyl-formamide and up to 30% of its recurring units derivedfrom vinylsulfonic acid.

5. The process of claim 1 wherein in said suspension is incorporated atleast a partially hydrolyzed polymer consisting essentially of recurringunits derived from N vinyl-N-methyl-formamide which has a molecularweight of about 20,000 to 5,000,000.

References Cited UNITED STATES PATENTS 3,194,727 7/1965 Adams 162-168FOREIGN PATENTS 1,082,016 9/1967 Great Britain 26094.7N

S. LEON BASHORE, Primary Examiner R. H. ANDERSON, Assistant Examiner US.Cl. X.R. 260-89.7

